Catalysis is a powerful, general phenomenon that allows a reaction to proceed rapidly at a relatively low temperature. A catalyst is a substance that speeds up a reaction without being consumed itself. Catalysts for chemical reactions can be single atoms, atomic clusters, single molecules, organometallic compounds, surfaces, and biomolecules such as ribozymes and proteins. Catalysts operate by providing a lower-energy pathway for the reaction.
In order to provide a lower-energy pathway for a reaction. Successful catalysis depends on controlling the local environment of the reactants such that the molecule is shifted to a more reactive state. In practice, this usually means stressing the molecular bonds, increasing the polarization around the molecule, holding a normally high-energy state stable for a longer time, or in some cases, simply increasing the number of times that a molecule attempts to react.
One important biological catalyst is the enzyme. Enzymes are a class of protein macromolecules that increase the rate of the majority of the biochemical reactions carried out at body temperature. The catalytic activity of the enzymes is reflected in their ability to precisely position molecules and atoms in reactive arrangements.
Surfaces of solid materials are an additional class of important catalysts. Solid surfaces adsorb gaseous reactants and weaken their intramolecular bonds. Weakening the molecular bonds in the reactants, lowers the activation energy needed for the reaction, thus increasing the rate of the reaction.
In order to control the energy of a reactant, the catalyst itself must attain certain morphology. The catalyst morphology can be governed by the orientation of one molecule or by the configuration of two or more neighboring molecules. In a single molecular layer, specific orientation of a molecule with respect to neighboring molecules may be obtained. A single layer of molecules that is adsorbed on a surface is sometimes called a monolayer. Monolayer-forming molecules are usually long-chain hydrocarbon molecules terminated with reactive groups. One end of the molecule is reactive to the solid surface such that when the molecules are introduced to the surface, they bind to it and assemble into a single molecular layer.
When a monolayer is produced with two different molecules, phase segregation can occur, leading to local domains, patches or worm-like phases in the monolayer. When a monolayer is constructed on the surface of a nanoparticle, phase segregation occurs as well. Nanoparticles possess a high radius of curvature and this high curvature leads to the generation of phase segregated domains that can be less than 1 or 2 nm. Thus, molecular domains on monolayer-coated nanoparticles may have molecular scale dimensions.
Enhancing the properties of a monolayer catalyst through variations in monolayer structure, improve the catalysis of desired reactions.